Honeycomb catalyst, denitration catalyst of denitration device, and exhaust gas denitration device

ABSTRACT

The present invention provides a honeycomb catalyst and an NO x  removal catalyst for use in an NO x  removal apparatus which can be employed at high efficiency, and a flue gas NO x  removal apparatus, whereby the running cost of a flue gas NO x  removal system in terms of the NO x  removal catalyst is reduced by about one-half. The honeycomb catalyst having gas conduits for feeding a gas to be treated from an inlet to an outlet of each conduit and performing gas treatment on the sidewalls of the conduit, wherein the honeycomb catalyst has an approximate length such that the flow of the gas to be treated which has been fed into the gas conduits is straightened in the vicinity of the outlet.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.10/540,250, which is a 371 of PCT Application No. PCT/JP2003/16773 filedDec. 25, 2003 and which claims benefit of JPA No. 2002-380831 filed Dec.27, 2002. The above-noted applications are incorporated herein byreference in their entirety.

TECHNICAL FIELD

The present invention relates to a honeycomb-form catalyst (hereinafterreferred to simply as honeycomb catalyst) for use in treatment ofautomobile exhaust gas, purification of gas, chemical synthesis, etc.More particularly, the invention relates to a high-performance NO_(x)removal catalyst and a flue gas NO_(x) removal apparatus, forefficiently removing NO_(x) from flue gas produced by a facility such asa thermal power station.

BACKGROUND ART

Conventionally, boilers provided in thermal power stations and a varietyof large-scale boilers employing a fuel such as petroleum, coal, or fuelgas, waste incinerators, and similar apparatuses have been equipped witha flue gas NO_(x) removal apparatus for treating exhaust gas whichapparatus contains a plurality of NO_(x) removal catalyst layers.

The NO_(x) removal catalyst is generally composed of a carrier (e.g.,TiO₂), an active component (e.g., V₂O₅), and a co-catalyst component(e.g., tungsten oxide or molybdenum oxide), and multi-component oxideNO_(x) removal catalysts such as VO_(x)-WO_(y)-TiO₂ andVO_(x)-MoO_(y)-TiO₂ are employed.

The NO_(x) removal catalysts typically assume the form of honeycomb,plate, etc. Honeycomb catalysts include a coated catalyst, which isfabricated by producing a honeycomb substrate and coating the substratewith a catalyst component; a kneaded catalyst, which is fabricated bykneading a substrate material with a catalyst component and molding intoa honeycomb catalyst; and an impregnated catalyst, which is fabricatedby impregnating a honeycomb substrate with a catalyst component.Plate-form catalyst are fabricated by coating a metallic substrate or aceramic substrate with a catalyst component.

In any case, during use, the catalytic performance of the abovecatalysts is problematically deteriorated with elapse of time as aresult of deposition, on the surface of the catalysts, of a substancewhich deteriorates the catalytic performance (hereinafter referred to asdeteriorating substance) or through migration of the dissolveddeteriorating substance into the catalysts.

In this connection, a variety of methods for regenerating an NO_(x)removal catalyst has conventionally been studied.

For example, there have been studied some methods including physicallyremoving a deteriorated portion and foreign matter so as to expose acatalytically active surface; e.g., a method including abrasion of aninner surface of a discharge gas conduit by use of an abrasive (see, forexample, Patent Document 1); a method including scraping a deterioratedsurface portion of an NO_(x) removal catalyst to thereby expose acatalytically active new surface (see, for example, Patent Document 2);and a method including causing a gas accompanying microparticles to flowthrough a through-hole to thereby remove foreign matter (see, forexample, Patent Document 3).

In addition, there have been studied catalytic performance regenerationmethods through washing; e.g., a method including washing a deterioratedcatalyst with an acid (pH≦5) or an alkali (pH≧8) (see, for example,Patent Document 4); a method including washing a deteriorated catalystsequentially with water or a dilute aqueous inorganic acid solution,with a 0.1 to 5 wt. % aqueous oxalic acid solution, and with water toremove oxalic acid residing on the catalyst (see, for example, PatentDocument 5); and a method including washing a deteriorated catalyst withwater (50° C. to 80° C.) followed by drying (see, for example, PatentDocument 6).

As described above, a variety of regeneration methods have been studied.However, regarding NO_(x) removal catalysts per se, the performance andspecifications thereof remain unchanged.

[Patent Document 1]

Japanese Patent Application Laid-Open (kokai) No. 1-119343 Claims andother sections)

[Patent Document 2]

Japanese Patent Application Laid-Open (kokai) No. 4-197451

[Patent Document 3]

Japanese Patent Application Laid-Open (kokai) No. 7-116523

[Patent Document 4]

Japanese Patent Application Laid-Open (kokai) No. 64-80444

[Patent Document 5]

Japanese Patent Application Laid-Open (kokai) No. 7-222924

[Patent Document 6]

Japanese Patent Application Laid-Open (kokai) No. 8-196920

DISCLOSURE OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide a honeycomb catalyst which facilitates detection of actuallydeteriorated NO_(x) removal catalysts, thereby attaining effectiveutilization of NO_(x) removal catalysts. Another object of the inventionis to provide an NO_(x) removal catalyst for use in an NO_(x) removalapparatus of the honeycomb catalyst. Still another object of theinvention is to provide a flue gas NO_(x) removal apparatus.

Accordingly, a first mode of the present invention for attaining theaforementioned objects provides a honeycomb catalyst having gas conduitsfor feeding a gas to be treated from an inlet to an outlet of eachconduit and performing gas treatment on the sidewalls of the conduit,characterized in that the honeycomb catalyst has an approximate lengthsuch that the flow of the gas to be treated which has been fed into thegas conduits is regulated and straightened in the vicinity of theoutlet.

According to the first mode, an exhaust gas fed through the inlet of thehoneycomb catalyst via the gas conduits is effectively caused to be incontact with the sidewalls until the flow of the gas is straightened,whereby catalytic reaction can be performed effectively. Thus, thehoneycomb catalyst is capable of performing catalytic reaction from theinlet to a portion in the vicinity of the outlet.

A second mode of the present invention is drawn to a specific embodimentof the honeycomb catalyst of the first mode, wherein the length Lb (mm)is represented by equation (A):

Lb=a(Ly/Lys·22e ^(0.035 (Ly·Uin)))  (A)

(wherein Uin (m/s) represents a gas inflow rate, Ly (mm) represents anaperture size, Lys is an aperture size of 6 mm (constant value), and “a”is a constant falling within a range of 3 to 6, when the aperture size(Ly) is 6 mm and the gas inflow rate is 6 m/s).

According to the second mode, the optimum length of the NO_(x) removalcatalyst so as to cause the catalyst to be involved in NO_(x) removalreaction throughout the length thereof can be reliably and preciselyspecified.

A third mode of the present invention provides an NO_(x) removalcatalyst for use in an NO_(x) removal apparatus, which is a honeycombcatalyst for use in a flue gas NO_(x) removal apparatus, the catalysthaving gas conduits for feeding an exhaust gas from an inlet to anoutlet of each conduit and performing NO_(x) removal on the sidewalls ofthe conduit, characterized in that the NO_(x) removal catalyst has anapproximate length such that the flow of the exhaust gas which has beenfed into the gas conduits is straightened in the vicinity of the outlet.

According to the third mode, an exhaust gas fed through the inlet of theNO_(x) removal catalyst via the gas conduits is effectively caused to bein contact with the sidewalls until the flow of the gas is straightened,whereby NO_(x) removal reaction can be performed effectively. Thus, theNO_(x) removal catalyst is capable of performing catalytic reaction fromthe inlet to a portion in the vicinity of the outlet.

A fourth mode of the present invention is drawn to a specific embodimentof the NO_(x) removal catalyst of the third mode for use in an NO_(x)removal apparatus, wherein the length Lb (mm) is represented by equation(A):

Lb=a(Ly/Lys·22e ^(0.035 (Ly-Uin)))  (A)

(wherein Uin (m/s) represents a gas inflow rate, Ly (mm) represents anaperture size, Lys is an aperture size of 6 mm (constant value), and “a”is a constant falling within a range of 3 to 6, when the aperture size(Ly) is 6 mm and the gas inflow rate is 6 m/s).

According to the fourth mode, the optimum length of the NO_(x) removalcatalyst so as to cause the catalyst to be involved in NO_(x) removalreaction throughout the length thereof can be reliably and preciselyspecified.

A fifth mode of the present invention is drawn to a specific embodimentof the NO_(x) removal catalyst of the third mode for use in an NO_(x)removal apparatus, wherein the length of the NO_(x) removal catalystfalls within a range of 300 mm to 450 mm.

According to the fifth mode, the catalyst is involved in NO_(x) removalreaction throughout the entire length thereof.

A sixth mode of the present invention provides a flue gas NO_(x) removalapparatus comprising a plurality of NO_(x) removal catalyst layersprovided in the gas flow direction, each catalyst layer being composedof a plurality of honeycomb NO_(x) removal catalysts juxtaposed in adirection crossing the gas flow direction,

each honeycomb NO_(x) removal catalyst having gas conduits for feedingan exhaust gas from an inlet to an outlet of each conduit and performingNO_(x) removal on the sidewalls of the conduit,

characterized in that each of the NO_(x) removal catalysts forming eachNO_(x) removal catalyst layer has an approximate length such that theflow of the exhaust gas which has been fed into the gas conduits isstraightened in the vicinity of the outlet, and two NO_(x) removalcatalyst layers adjacent to each other are disposed with a spacetherebetween, the space serving as a common gas conduit where exhaustgas flows discharged through the NO_(x) removal catalysts areintermingled one another.

According to the sixth mode, the flow of an exhaust gas fed through theinlets of the NO_(x) removal catalyst layers via the gas conduits is notstraightened to a portion in the vicinity of the outlet and iseffectively caused to be in contact with the sidewalls, whereby NO_(x)removal reaction can be performed effectively. The exhaust gas flowdischarged through each NO_(x) removal catalyst layer forms turbulentflow in each common gas conduit, and the turbulent flow is introduced toa subsequent NO_(x) removal catalyst layer. Thus, the entirety of thesubsequent NO_(x) removal catalyst can also be effectively involved inNO_(x) removal reaction.

A seventh mode of the present invention is drawn to a specificembodiment of the flue gas NO_(x) removal apparatus of the sixth mode,wherein the length Lb (mm) is represented by equation (A):

Lb=a(Ly/Lys·22e ^(0.035 (Ly-Uin)))  (A)

(wherein Uin (m/s) represents a gas inflow rate, Ly (mm) represents anaperture size, Lys is an aperture size of 6 mm (constant value), and “a”is a constant falling within a range of 3 to 6, when the aperture size(Ly) is 6 mm and the gas inflow rate is 6 m/s)

According to the seventh mode, the optimum length of the NO_(x) removalcatalyst so as to cause the catalyst to be involved in NO_(x) removalreaction throughout the length thereof can be reliably and preciselyspecified.

An eighth mode of the present invention is drawn to a specificembodiment of the flue gas NO_(x) removal apparatus of the sixth mode,wherein the length of the NO_(x) removal catalyst falls within a rangeof 300 mm to 450 mm.

According to the eighth mode, the catalyst is involved in NO_(x) removalreaction throughout the entire length thereof.

A ninth mode of the present invention is drawn to a specific embodimentof the flue gas NO_(x) removal apparatus of the seventh or eighth mode,which has 3 to 5 stages of the NO_(x) removal catalyst layers having aspecific length (Lb).

According to the ninth mode, all of the provided NO_(x) removal catalystlayers are effectively involved in NO_(x) removal reaction.

The present invention is applicable to any type of conventionallyemployed honeycomb catalyst. The term “honeycomb catalyst” refers to acatalyst unit including gas conduits having a cross-section of a polygonsuch as square, hexagon, or triangle, and performing catalytic reactionon the sidewalls of the gas conduits. No particular limitation isimposed on the form of the honeycomb catalyst, and typical forms includea cylinder containing gas conduits each having a hexagonalcross-section, and a rectangular prism containing gas conduits eachhaving a square cross-section and arranged in a lattice-like form.

Conventionally, typical honeycomb NO_(x) removal catalysts have a gasconduit pitch of 7 mm (aperture size: about 6 mm) and a length of about700 mm to 1,000 mm. The present inventors have investigated thedeterioration status of such catalysts after use along a longitudinaldirection, and have found that the catalysts are more deteriorated onthe inlet side than on the outlet side; the deterioration status isvirtually unchanged in a portion ranging from the 300 mm site from theinlet to the outlet; and particularly, the catalysts are less involvedin NO_(x) removal reaction in a portion ranging from the outlet to the300 mm site (from the outlet) than in a portion on the inlet side. Thepresent invention has been accomplished on the basis of these findings.In other words, the present invention has been accomplished on the basisof the following finding by the inventors. Specifically, an exhaust gasis fed into an NO_(x) removal catalyst through gas conduits as aturbulent flow, and NO_(x) removal reaction is performed through contactof the gas with the sidewalls of the gas conduits. However, the flow ofthe thus-reacted exhaust gas is gradually straightened. Contact of thestraightened gas flows with the sidewalls of the conduits is minimized,thereby failing to attain effective NO_(x) removal.

Furthermore, one conceivable mechanism that explains reduction inNO_(x)- or NH₃-removal efficiency is as follows. When an exhaust gas isfed from a wide space on the upstream side of the NO_(x) removalcatalyst to gas conduits of the catalyst, percent space of the gas isreduced from 1 to 0.6 to 0.7. The exhaust gas passes through the gasconduits while being in contact with the sidewalls of the conduits(catalyst surfaces) in a considerably turbulent state. However, duringthe course of passage through the conduits, the gas flows are graduallyregulated and straightened and mass transfer is controlled throughdiffusion only. After straightening, NO_(x) molecules and NH₃ moleculeswhich are to collide with the sidewalls decrease in number considerably.

Thus, when an NO_(x) removal catalyst including gas conduits each havingan aperture size of 6 mm (pitch: about 7 mm) is used, the flow ofintroduced exhaust gas is straightened at a depth of about 300 to 450 mmfrom the inlet, although the depth varies depending on the flowconditions of the exhaust gas. According to the present invention,NO_(x) removal catalysts each having a length of about 300 to 450 mm areincorporated into a flue gas NO_(x) removal apparatus. The length issuitable for attaining effective utilization of the NO_(x) catalysts,and NO_(x) removal performance is unchanged, even though the length ofthe catalysts increases. As compared with conventional, typical cases inwhich two stages of NO_(x) removal catalysts each having a length of 700mm to 1,000 mm are employed, use of three stages of NO_(x) removalcatalysts each having a length of 400 mm to 500 mm or use of four ormore stages of NO_(x) removal catalysts each having a length of about300 mm remarkably enhances NO_(x) removal performance. Preferably, twoNO_(x) removal catalyst layers adjacent to each other are disposed witha space therebetween, the space serving as a common gas conduit whereexhaust gas flows that are to be treated and that are discharged throughthe NO_(x) removal catalysts are intermingled one another. The length ofthe common gas conduit is preferably such that turbulent flow issatisfactorily formed. Needless to say, a baffle plate or a similarmember for intentionally forming turbulent flow may be provided in thecommon gas conduit.

Meanwhile, NO_(x) removal by use of an NO_(x) removal catalyst isperformed at an exhaust gas flow rate of about 5 m/sec to 10 m/sec, anda honeycomb catalyst is considered to provide the same NO_(x) removaleffect when used under such a flow rate.

In the honeycomb catalyst of the present invention, catalytic reactionoccurs on the sidewalls of the honeycomb structure. Thus, the honeycombcatalyst may be employed not only as an NO_(x) removal catalyst for usein a flue gas NO_(x) removal apparatus, but also as a type of catalystfor any purpose, so long as the catalyst has structural characteristicssuch that fluid to be treated passes through the honeycomb. Inparticular, the honeycomb catalyst is applicable to any case where thefluid to be reacted contains a substance that deteriorates the catalystto reduce reaction efficiency.

As described hereinabove, the present invention provides a honeycombcatalyst and an NO_(x) removal catalyst for use in an NO_(x) removalapparatus which can be employed at high efficiency, and a flue gasNO_(x) removal apparatus, whereby the running cost of a flue gas NO_(x)removal system in terms of the NO_(x) removal catalyst is reduced byabout one-half.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a configuration of a flue gas NO_(x) removalapparatus employing an NO_(x) removal catalyst management unit accordingto one embodiment of the present invention.

FIG. 2 is a graph showing the results of Test Example 1 of the presentinvention.

FIG. 3 is a graph showing the results of Test Example 2 of the presentinvention.

FIG. 4 is a graph showing the results of Test Example 2 the presentinvention.

FIG. 5 is a graph showing the results of Test Example 3 the presentinvention.

FIG. 6 is a graph showing the results of Test Example 4 the presentinvention.

FIG. 7 is a graph showing the results of Test Example 4 the presentinvention.

FIG. 8 is a graph showing the results of Test Example 5 the presentinvention.

FIG. 9 is a graph showing the results of Test Example 6 the presentinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

Best modes for carrying out the invention will next be described withreference to the FIGS. The description is made only for the illustrationpurpose, and should not be construed as limiting the invention thereto.The present embodiment is the case in which a honeycomb catalyst isemployed as an NO_(x) removal catalyst used in a flue gas NO_(x) removalapparatus. Needless to say, the present invention is not limited to suchuse.

Embodiment

FIG. 1 schematically shows a configuration of a flue gas NO_(x) removalapparatus equipped with an NO_(x) removal catalyst according to oneembodiment of the present invention. Actually, the flue gas NO_(x)removal apparatus is provided in a thermal power station. However, noparticular limitation is imposed on the facility that includes theNO_(x) removal catalyst management unit of the embodiment.

As shown in FIG. 1, a flue gas NO_(x) removal apparatus 10 includes anexhaust duct 12 and a treated gas duct 13. The exhaust duct 12 is incommunication with a boiler unit installed in a thermal power stationthat is connected with an apparatus body 11 on the upstream side. Thetreated gas duct 13 is connected with the apparatus body 11 on thedownstream side. In the apparatus body 11, a plurality of NO_(x) removalcatalyst layers (4 layers in this embodiment) 14A to 14D are disposed atpredetermined intervals. The NO_(x) removal catalyst layers 14A to 14Dare arranged so that a discharge gas introduced through the exhaust duct12 is sequentially passed therethrough, and reduce the level of nitrogenoxide (NO_(x)) of the discharge gas through contact with the dischargegas passing through the catalyst layers. Notably, to the exhaust duct 12communicating with the boiler unit, NH₃ is injected in an amount inaccordance with the amount of the discharge gas fed from the boilerbody.

No particular limitation is imposed on the type, shape, etc. of theNO_(x) removal catalysts 14 forming the NO_(x) removal catalyst layers14A to 14D. Generally, each catalyst is composed of TiO₂ serving as acarrier and V₂O₅ serving as an active component. In this embodiment,honeycomb catalysts were employed. In the present embodiment, eachcatalyst layer employs a catalyst in the form of columnar honeycomb, anda plurality of honeycomb catalysts are juxtaposed in combination,thereby forming the catalyst layers 14A to 14D. Each NO_(x) removalcatalyst 14 has a length of 350 mm and includes a plurality of gasconduits 14 a arranged at pitches of 7 mm. The interlayer spacingbetween two adjacent NO_(x) removal catalyst layers 14A to 14D is about2,000 mm, which corresponds to the height for allowing a person toperform inspection or sampling of a catalyst. Each interlayer spaceserves as a common gas conduit 19.

An NO_(x) removal catalyst management unit 20 is provided with gassampling means 15A through 15E on the inlet and outlet sides ofrespective NO_(x) removal catalyst layers 14A through 14D. The gassampling means 15A through 15E are connected with NO_(x) concentrationmeasurement means 16A through 16E and with NH₃ concentration measurementmeans 17A through 17E. The data obtained by the measurement means aretransferred to a percent NO_(x) removal determination means 18 forcalculating percent NO_(x) removal and percent NO_(x) removalcontribution of the respective NO_(x) removal catalyst layers 14Athrough 14D.

The gas sampling means 15A through 15E sample, via sampling tubes, a gasto be sampled in a desired amount and at a desired timing, andsubsequently feed the sampled gas to the NO_(x) concentrationmeasurement means 16A through 16E and to the NH₃ concentrationmeasurement means 17A through 17E.

No particular limitation is imposed on the timing for sampling a gas bythe gas sampling means 15A through 15E. Generally, sampling is carriedout during usual operation of the power station, preferably at thenominal load where the amount of gas reaches the maximum, if possible.The interval between sampling operations may be prolonged to about sixmonths, and the interval is sufficient for managing the performance ofthe NO_(x) removal catalyst layers 14A through 14D. However, if theinterval is shortened, precision in management is enhanced. Thus, thesampling is preferably carried out, for example, once every one to twomonths. Particularly, in a catalyst layer placed on the downstream side,variation of obtained data increases due to decrease in NH₃concentration. Thus, in order to attain better management andevaluation, preferably, determination of NH₃ concentration is performedat short intervals, and percent NO_(x) removal is calculated from anaveraged NH₃ concentration value.

The percent NO_(x) removal determination means 18 collects themeasurement data from the NO_(x) concentration measurement means 16Athrough 16E and the NH₃ concentration measurement means 17A through 17E,and calculates, from the measurement data, percent NO_(x) removal andpercent NO_(x) removal contribution of respective NO_(x) removalcatalyst layers 14A through 14D.

On the basis of an inlet mole ratio (i.e., inlet NH₃/inlet NO_(x)) ofthe NO_(x) removal catalyst layers 14A through 14D, theNH₃-concentration-based percent NO_(x) removal (η) is determined fromthe following equation (1):

η={(inlet NH₃−outlet NH₃)/(inlet NH₃−outlet NH₃+outlet NO_(x))}×100×(evaluation mole ratio/inlet mole ratio)  (1)

As used herein, the term “evaluation mole ratio” refers to a mole ratiowhich is predetermined for the purpose of evaluating an NO_(x) removalcatalyst. The evaluation mole ratio may be predetermined to an arbitraryvalue; for example, 0.8, which is almost equal to a mole ratio typicallyemployed for operating a power station.

COMPARATIVE EXAMPLE

The procedure of Example was repeated, except that the length of eachNO_(x) removal catalyst was changed to 860 mm, to thereby provide a fluegas NO_(x) removal apparatus.

TEST EXAMPLE 1

From an NO_(x) removal catalyst layer which had been used for 50,000hours in the apparatus of Comparative Example, catalyst portions (20 mmsite to 850 mm site, from the inlet) were sampled in the longitudinaldirection. TiO₂ concentration and concentrations of catalystdeterioration substances (CaO and SO₃) on the surface of each catalystsample were determined.

Catalyst portions (50 mm×50 mm×100 mm in length) were cut from acatalyst included in each catalyst layer, and set in a performance testmachine. Portions at the 100 mm site, the 450 mm site, and the 800 mmsite were tested. The test gas was fed at a mole ratio (inlet moleratio=inlet NH₃/inlet NO_(x)) of 0.82 and an AV (amount of treatable gasper unit surface area of the catalyst) of 6.5, and percent NO_(x)removal η was calculated on the basis of the aforementioned formula (1)employing NH₃ concentration.

The results are shown in FIG. 2. As a reference product, a new (unused)catalyst was also measured in terms of percent NO_(x) removal.

The results indicate that the catalyst was severely deteriorated in aportion ranging from the inlet to the 300 mm site, and that a portionranging from the 450 mm to the outlet exhibits percent NO_(x) removalalmost equal to that of a new catalyst product.

TEST EXAMPLE 2

An NO_(x) removal catalyst which had been used for 28,000 hours, afterregeneration through washing with water, in the apparatus of ComparativeExample, was re-installed in the flue gas NO_(x) removal apparatus suchthat the catalyst was inverted with respect to the direction of the flowof discharge gas. FIG. 3 shows the results.

The results indicate that the inverted catalyst exhibits NO_(x) removalperformance approximately equal to that of a new catalyst product.

After regeneration and use for 30,000 hours, the inverted catalyst wasinvestigated in terms of change in percent NO_(x) removal. The resultsare shown in FIG. 4. As is clear from FIG. 4, a portion on the outletside of the catalyst was not deteriorated and maintained performance ashigh. as that of a new catalyst product. The portion per se was found toexhibit sufficient NO_(x) removal performance.

TEST EXAMPLE 3

The NO_(x) removal which had been used in the apparatus of ComparativeExample was cut at the 600 mm site from the inlet (along thelongitudinal direction), and the cut catalyst piece was set in aperformance test machine. Percent NO_(x) removal was determined at aplurality of sites at intervals of 100 mm under the followingconditions: mole ratios (i.e., inlet mole ratio=inlet NH₃/inlet NO_(x))of 0.6, 0.8, 1.0, and 1.2; 360° C.; and fluid inflow rate of 6 m/s. Theresults are shown in Table 1 and FIG. 5.

The results indicate that percent NO_(x) removal tends to increase inproportion to the distance from the inlet (i.e., length of the catalyst)and that the increase in percent NO_(x) removal tends to be suppressedwhen the catalyst length exceeds a certain value. The tendency isattributable to the flow of exhaust gas being gradually straightened.

TABLE 1 100 200 300 400 500 600 0.6 17.7 30.4 39.5 46.1 50.8 54.2 0.821.3 36.9 48.3 56.7 62.9 67.4 1.0 23.2 40.5 53.5 63.2 70.5 75.9 1.2 24.042.0 55.4 65.4 73.0 78.6

TEST EXAMPLE 4

A honeycomb catalyst (600 mm×6 mm×6 mm, aperture size: 6 mm (pitch: 7mm)) was subjected to simulation under the following conditions: 350° C.and fluid inflow rate (Uin): 4, 6, and 10 m/s.

The simulation results of the honeycomb catalyst indicate that Uin andthe distance from the inlet to a site where turbulent flow energy islost in the course of transition from turbulent flow to laminar flow(hereinafter referred to as sustained turbulent flow distance (Lts))have the relationship shown in FIG. 6. Specifically, sustained turbulentflow distance (Lts) values at fluid inflow rates (Uin) of 4, 6, and 10m/s were calculated as 50, 80, and 180 mm, respectively.

Theoretically, conditions of fluid are generally determined from inflowrate (Uin) and Reynolds number Re; i.e., a parameter employing aperturesize Ly (Re=Uin·Ly/v, v=5.67×10⁻⁵ m²/S; constant).

In a honeycomb catalyst having an aperture size of 6 mm, sustainedturbulent flow distance Lts (mm) is derived from a product of inflowrate Uins (m/s) and aperture size Lys (mm). Thus, the relationshipbetween sustained turbulent flow distance Lts and a product of inflowrate Uins (Uin) and aperture size Lys (Ly), as shown in FIG. 6, wasobtained. Through the least squares method, sustained turbulent flowdistance Lts at an aperture size (Lys) of 6 mm is approximatelyrepresented by the following equation (2).

Lts=22e^(0.035 (Lys·Uins))  (2)

When the aperture size Lys is 6 mm (constant value), the aperture sizeLy (mm) is an arbitrary parameter, and Uin (m/s) represents a gas inflowrate, sustained turbulent flow distance Lt can be represented by thefollowing formula (3), which is a general equation.

Lt=Ly/Lys·22e ^(0.035(Ly·Uin))  (3)

The simulation results were compared with the approximate length(optimum length) of the actual catalyst, the length being such that theflow of the exhaust gas fed into the gas conduits is straightened.Specifically, the relationship between sustained turbulent flow distanceLt and the optimum length of an actual catalyst (i.e., the length of astained portion of the catalyst (stain length), which is an index fordetecting straightening) was investigated. As shown in FIG. 7, in anactual stage of the employed apparatus, turbulent flow is maintainedover a portion of the catalyst having a distance longer than thesustained turbulent flow distance Lt, which is derived throughsimulation. One possible reason of this phenomenon is that inflow rateis varied and flow of the fluid is disturbed.

Accordingly, in an actual catalyst unit, the distance from the inlet toa site where straightening starts (i.e., the optimum catalyst length)must be determined from the above stain length and a certain safetylength. Specifically, equation (3) must be multiplied by a constant “a,”and the optimum length Lb of the catalyst is considered to berepresented by the following equation (4). Note that “a” is a constantfalling within a range of 3 to 6, when the aperture size of a honeycombcatalyst is 6 mm (pitch: 7 mm) and the gas inflow rate is 6 m/s.

Lb=a·Lt  (4)

In the aforementioned Test Example 1, a honeycomb catalyst having anaperture size of 6 mm (pitch: 7 mm) was used at a gas inflow rate of 6m/s. Thus, Lt is 80 mm. When the constant “a” is adjusted to about 3.8,Lt is about 300 mm, which corresponds to the length of a severelydeteriorated portion of the catalyst, whereas when the constant “a” isadjusted to about 5.6, Lt is about 450 mm, which corresponds to thelength of a portion of the catalyst including a portion exhibitingcatalytic performance equivalent to that of a new catalyst product.

In the same honeycomb catalyst, when “a” falls within a range of 3 to 6,the optimum length Lb falls within a range of about 240 to 480 mm. Therange of Lb virtually coincides with a range of about 300 to 450 mm,which is considered to be a catalyst length which allows the exhaust gasin the gas conduits starts straightening of the flow. Thus, the optimumlength Lb is selected from the range of 240 to 480 mm, corresponding tothe “a” value of 3 to 6.

TEST EXAMPLE 5

The concept and equation (4) about the optimum length Lb, which wereobtained in Test Example 4, was confirmed in apparatus design.Specifically, a variety of catalyst layer sets having different catalystlengths and stage numbers were analyzed in terms of percent overallNO_(x) removal and unreacted NH₃ through a conventional apparatusdesigning method on the basis of an SV value (amount of treatable gasper unit volume of the catalyst) and an AV value (amount of treatablegas per unit surface area of the catalyst). The catalyst layer sets(length and number of layers) are as follows: Pattern 1 (in Table 2);catalyst length 1,000 mm, 1 stage, Pattern 2 (in Table 2); catalystlength 500 mm, 2 stages, Pattern 3 (in Table 2); catalyst length 333 mm,3 stages, Pattern 4 (in Table 2); catalyst length 250 mm, 4 stages, andPattern 5 (in Table 2); catalyst length 200 mm, 5 stages. The evaluationresults of the catalyst sets are shown in Table 2 and FIG. 8.

The results indicate that, even when the total catalyst length is thesame, a multi-stage catalyst exhibits an enhanced percent NO_(x)removal, and that a catalyst set (catalyst length 250 mm, 4 stages)exhibited the highest overall percent NO_(x) removal. As compared withthe case of a catalyst (catalyst length 1,000 mm, 1 stage) (percentNO_(x) removal: 84.3%), a catalyst set (catalyst length 250 mm, 4stages), the percent NO_(x) removal was as high as 90%. In this case,unreacted NH₃ was minimized. As a result, when a honeycomb catalysthaving an aperture size of 6 mm (pitch: 7 mm) is used at a gas inflowrate of 6 m/s, the optimum length thereof is approximately 250 mm, whichfalls within the optimum length Lb range of 240 mm to 480 mm, calculatedby equation (4).

In addition, when three to five stages of catalyst layers having alength almost equivalent to that of the optimum length Lb are provided,high overall percent NO_(x) removal was found to be attained.

TABLE 2 Pattern 1 2 3 4 5 SV (m₃N/h · m³) 5,950 5,950 5,950 5,950 5,950AV (m₃N/h · m²) 14.9 14.9 14.9 14.9 14.9 Catalyst length (mm) 1,000 500333 250 200 Inlet NO_(x) (ppm) 300 300 300 300 300 Inflow mole ratio0.95 0.95 0.95 0.95 0.95 Inlet NH₃ (ppm) 285 285 285 285 285 Stage 1NO_(x) removal (%) 84.3 68.6 56.0 46.9 39.6 Outlet NO_(x) (ppm) 47 94132 159 181 Outlet NH₃ (ppm) 32 79 117 144 166 Mole ratio 0.68 0.84 0.890.91 0.92 Stage 2 NO_(x) removal (%) 64.4 54.2 45.9 39.0 Outlet NO_(x)(ppm) 34 61 86 110 Outlet NH₃ (ppm) 19 46 71 95 Mole ratio 0.75 0.830.86 Stage 3 NO_(x) removal (%) 49.5 44.1 38.1 Outlet NO_(x) (ppm) 31 4868 Outlet NH₃ (ppm) 16 33 53 Mole ratio 0.69 0.78 Stage 4 NO_(x) removal(%) 39.2 36.3 Outlet NO_(x) (ppm) 29 44 Outlet NH₃ (ppm) 14 29 Moleratio 0.66 Stage 5 NO_(x) removal (%) 32.2 Outlet NO_(x) (ppm) 30 OutletNH₃ (ppm) 15 Apparatus outlet NO_(x) (ppm) 47.1 33.5 30.6 29.2 29.6Overall NO_(x) removal (%) 84.3 88.8 89.8 90.3 90.1 Unreacted NH₃ (ppm)32 19 16 14 15

TEST EXAMPLE 6

In a manner similar to Test Example 5, the catalyst layer sets (lengthand type of catalyst layer(s)) shown in Test Example 5 were analyzed interms of apparatus outlet NO_(x) and unreacted NH₃ through aconventional apparatus designing method under the conditions: inletNO_(x)=1,000 ppm, inflow mole ratio=0.83, and inlet NH₃=830 ppm). Theresults are shown in Table 3 and FIG. 9.

The results indicate that a catalyst set (catalyst length 250 mm, 4stages) exhibited the lowest apparatus outlet NO_(x) and unreacted NH₃.Therefore, a honeycomb catalyst having a length of 250 mm was found toeffectively work in an apparatus where high concentration NO_(x) must betreated (e.g., NO_(x) removal apparatus for a diesel engine).

TABLE 3 Pattern 1 2 3 4 5 SV (m₃N/h · m³) 5,950 5,950 5,950 5,950 5,950AV (m₃N/h · m²) 14.9 14.9 14.9 14.9 14.9 Catalyst length (mm) 1,000 500333 250 200 Inlet NO_(x) (ppm) 1,000 1,000 1,000 1,000 1,000 Inflow moleratio 0.83 0.83 0.83 0.83 0.83 Inlet NH₃ (ppm) 830 830 830 830 830 Stage1 NO_(x) removal (%) 77.9 64.0 52.6 44.2 37.4 Outlet NO_(x) (ppm) 221360 474 558 626 Outlet NH₃ (ppm) 51 190 304 388 456 Mole ratio 0.23 0.530.64 0.70 0.73 Stage 2 NO_(x) removal (%) 44.7 44.2 39.5 34.6 OutletNO_(x) (ppm) 199 265 337 409 Outlet NH₃ (ppm) 29 95 167 239 Mole ratio0.36 0.50 0.58 Stage 3 NO_(x) removal (%) 25.2 29.6 29.6 Outlet NO_(x)(ppm) 198 238 288 Outlet NH₃ (ppm) 28 68 118 Mole ratio 0.28 0.41 Stage4 NO_(x) removal (%) 17.0 20.8 Outlet NO_(x) (ppm) 197 228 Outlet NH₃(ppm) 27 58 Mole ratio 0.26 Stage 5 NO_(x) removal (%) 12.9 OutletNO_(x) (ppm) 199 Outlet NH₃ (ppm) 29 Apparatus outlet NO_(x) (ppm) 221.5199.0 198.0 197.3 198.8 Overall NO_(x) removal (%) 77.9 80.1 80.2 80.380.1 Unreacted NH₃ (ppm) 51 29 28 27 29

TEST EXAMPLE 7

Two types of NO_(x) removal catalyst sets for a diesel engine wereprovided for removal of high concentration NO_(x). In one catalyst set,the first stage was divided to form a multi-stage, and no such divisionis performed with respect to the other catalyst set. In a manner similarto Test Example 5, apparatus outlet NO_(x), overall percent NO_(x)removal, and unreacted NH₃ were calculated through a conventionalapparatus designing method. The results are shown in Table 4.

As is clear from Table 4, as compared with the case in which the firststage remained undivided, the divided first stage (700 mm into 350 mm+350 mm), each divided stage having an optimum Lb, exhibited a slightlyreduced apparatus outlet NO_(x) and unreacted NH₃ and a slightlyenhanced overall percent NO_(x) removal. In other words, when a catalysthaving a length that is about double the optimum length Lb theaforementioned equation (4) is divided, all catalytic performancesincluding apparatus outlet NO_(x) overall percent NO_(x) removal, andunreacted NH₃ can be enhanced.

Therefore, in an apparatus employing an NO_(x) removal catalyst having alength twice or more the optimum length Lb, when the NO_(x) removalcatalyst is divided into sub-layers having an approximate optimum lengthLb, performance of the apparatus is considered to be enhanced. In TestExample 7, if the stage 2 catalyst layer and the stage 3 catalyst layer(shown in Table 4), each having a length of 700 mm, are divided intosub-layers having an approximate optimum length Lb, performance of theapparatus is considered to be surely enhanced.

TABLE 4 Nondivided Divided stage stages SV (m₃N/h · m³) 5,950 5,950 AV(m₃N/h · m²) 14.9 14.9 Catalyst length/stage 1 (mm) 700 350 Catalystlength/stage 1 350 divided (mm) Catalyst length/stage 2 (mm) 700 700Catalyst length/stage 3 (mm) Catalyst Stage 2 3 Inlet NO_(x) (ppm) 1,0001,000 Inflow mole ratio 0.81 0.81 Inlet NH₃ (ppm) 810 810 Stage 1 NO_(x)removal (%) 71.2 53.5 Outlet NO_(x) (ppm) 288 465 Outlet NH₃ (ppm) 98275 Mole ratio 0.34 0.59 Stage 2 NO_(x) removal (%) 32.2 42.8 OutletNO_(x) (ppm) 195 266 Outlet NH₃ (ppm) 5 76 Mole ratio 0.29 Stage 3NO_(x) removal (%) 27.0 Outlet NO_(x) (ppm) 194 Outlet NH₃ (ppm) 4 Moleratio Apparatus outlet NO_(x) (ppm) 195.5 194.2 Overall NO_(x) removal(%) 80.5 80.6 Unreacted NH₃ (ppm) 5 4

INDUSTRIAL APPLICABILITY

The present invention is remarkably advantageous for a catalyst and anapparatus which are required to perform high-level NO_(x) removal andhigh-concentration NO_(x) removal treatment.

1-9. (canceled)
 10. A flue gas NO_(x) removal apparatus consisting of aplurality of NO_(x) removal catalyst layers provided in the gas flowdirection, each catalyst layer being composed of a plurality ofhoneycomb NO_(x) removal catalysts juxtaposed in a direction crossingthe gas flow direction, each honeycomb NO_(x) removal catalyst havinggas conduits including an aperture for feeding an exhaust gas from aninlet to an outlet of each conduit and performing NO_(x) removal on thesidewalls of the conduit, the gas conduits constituting the plurality ofthe NO_(x) removal catalyst layers each having approximately the sameaperture size, characterized in that each of the NO_(x) removalcatalysts forming each NO_(x) removal catalyst layer has an approximatelength such that the flow of the exhaust gas which has been fed into thegas conduits is straightened in the vicinity of the outlet, that thelength (Lb) is specified by a sustained turbulent flow distance (Lt)which is the distance from the inlet to a site where turbulent flowenergy is lost in the course of transition from turbulent flow tolaminar flow, and that two NO_(x) removal catalyst layers adjacent toeach other are disposed with a space therebetween, the space serving asa common gas conduit where exhaust gas flows discharged through theNO_(x) removal catalysts are intermingled one another.
 11. A flue gasNO_(x) removal apparatus according to claim 10, wherein the length Lb(mm) is represented by equation (A):Lb=a(Ly/Lys·22e ^(0.035(Ly·Uin)))  (A) (wherein Uin (m/s) represents agas inflow rate, Ly (mm) represents an aperture size, Lys is an aperturesize of 6 mm (constant value), and “a” is a constant falling within arange of 3 to 6, when the aperture size (Ly) is 6 mm and the gas inflowrate is 6 m/s).
 12. A flue gas NO_(x) removal apparatus according toclaim 10, wherein the length of the NO_(x) removal catalyst falls withina range of 300 mm to 450 mm.
 13. A flue gas NO_(x) removal apparatusaccording to claim 11, wherein 3 to 5 stages of the NO_(x) removalcatalyst layers each having a specific length (Lb) are provided.
 14. Aflue gas NO_(x) removal apparatus according to claim 12, wherein 3 to 5stages of the NO_(x) removal catalyst layers each having a specificlength (Lb) are provided.